Optical spectrum analyzer

ABSTRACT

An optical spectrum analyzer includes an optical section  130  for executing light dispersion into a spectrum and wavelength sweep for input measured light, converting the measured light into an electric signal, and outputting the electric signal, a control section  101  for controlling the wavelength sweep of the optical section and outputting a sampling clock of a period shifting from a cycle period of the measured light for each wavelength of the wavelength sweep, and a measurement section  140  for executing sequential sampling of the electric signal from the optical section for each sampling clock.

TECHNICAL FIELD

The present disclosure relates to an optical spectrum analyzer formeasuring the spectrum of measured light with a plurality of pulsesoccurring repeatedly in one frame period. More specifically, the presentdisclosure relates to an optical spectrum analyzer for enabling the userto observe instantaneous change in an optical spectrum.

RELATED ART

FIG. 8 is a drawing to show the configuration of a measurement system ofan optical spectrum analyzer according to a representative Czerny-Turnermonochromator system.

In FIG. 8, an optical fiber 10 has an incidence end on which measuredlight is incident, and transmits the incident measured light. Anemission end of the optical fiber 10 on the side of an optical section130 described later is connected to an incidence slit 131 in the opticalsection 130 described later.

A motor controller 110 outputs a ramp wave signal of a predeterminedwaveform for driving a motor 134 for rotating a diffraction grating 135described later, and has output connected to a divider 120.

The ramp wave signal of the predetermined waveform supplied from themotor controller 110 is input to the divider 120, which then divides theramp wave signal for output. The divided output is supplied to the motor134 and a signal processing section 160 described later.

The optical section 130 executes light dispersion into a spectrum andwavelength sweep for the measured light emitted from the output end ofthe optical fiber 10, converts the measured light into an electricsignal, and outputs the electric signal. It has the incidence slit 131,a concave mirror 132, the motor 134, the diffraction grating 135, aconcave mirror 136, an exit slit 137, and a photodetector 138.

The incidence slit 131 is placed in the proximity of the emission end ofthe optical fiber 10 for limiting a light flux so that the measuredlight having a predetermined emission angle, of the measured lightemitted from the emission end of the optical fiber 10 arrives at theconcave mirror 132.

The concave mirror 132 is a kind of collimator means for converting themeasured light having a spread angle from the optical fiber 10 incidentthrough the incidence slit 131 into collimated light, and reflects themeasured light converted into the collimated light toward thediffraction grating 135.

The motor 134 is a rotation drive source to which the ramp wave signalprovided by the divider 120 is supplied as a drive signal, and drivesthe diffraction grating 135 by rotation responsive to the ramp wavesignal.

The diffraction grating 135 is an optical element for executing lightdispersion of the diffraction angle responsive to the wavelength byreflection interference of a plurality of minute parallel grooves. Itreceives collimated measured light from the concave mirror 132 anddiffracts the dispersed measured light in the direction of the concavemirror 136.

The concave mirror 136 is a kind of light condensing means forcondensing dispersed diffraction light which is collimated light foreach wavelength from the diffraction grating 135. It condenses thediffraction light toward the photodetector 138.

The exit slit 137 is placed in the proximity of the light detection faceof the photodetector 138 for limiting a light flux so that the measuredlight of a predetermined wavelength, of the measured light dispersedthrough the diffraction grating 135 and condensed on the concave mirror136 toward the photodetector 138.

The photodetector 138 is a photoelectric conversion element fordetecting the measured light of the predetermined wavelength emittedthrough the exit slit 137 and outputting an electric signal, and hasoutput connected to an A-D converter 150.

The A-D converter 150 has input to which the output of the photodetector138 is connected, executes A-D conversion in response to a samplingclock, outputs a digital signal, and has output connected to the signalprocessing section 160.

A sampling clock generation section 152 generates a sampling clock of apredetermined frequency and outputs the sampling clock to the A-Dconverter 150.

The signal processing section 160 performs signal processing of thedigital signal from the A-D converter 150, the sampling result of themeasured light of the predetermined wavelength, with the ramp wavesignal provided by the divider 120 as a trigger, generates waveforminformation of the spectrum component of the measured light, and outputsthe waveform information to a waveform display 170.

The waveform information output from the signal processing section 160is input to the waveform display 170 for producing various displays ofthe waveform information in the forms of graphs and numeric values on adisplay screen.

In FIG. 8, measured light is incident on the optical section 130 fromthe emission end of the optical fiber 10. At this time, the incidenceslit 131 limits a light flux so that the measured light having apredetermined emission angle, of the measured light emitted from theemission end of the optical fiber 10 arrives at the concave mirror 132.

The measured light emitted from the emission end of the optical fiber 10and arriving at the concave mirror 132 arrives at the diffractiongrating 135 as collimated light.

At this time, the measured light diffracted through the diffractiongrating 135 is dispersed into a spectrum and is in a state in which thediffraction angle changes for each wavelength.

Therefore, the wavelength of the measured light diffracted through thediffraction grating 135 and then condensed on the concave mirror 136 andemitted through the exit slit 137 changes in synchronization withrotation of the diffraction grating 135 and wavelength sweep isrealized.

The measured light emitted through the exit slit 137 is detected at thephotodetector 138, which then executes photoelectric conversion of themeasured light into an electric signal. The electric signal detected andgenerated by the photodetector 138 is converted into a digital signal bythe A-D converter 150 driven according to a sampling clock from thesampling clock generation section 152.

The signal processing section 160 performs signal processing of thedigital signal with a ramp wave signal as a trigger and generateswaveform information of the spectrum component of the measured light,and the waveform information is displayed on the waveform display 170.That is, the emission timing through the exit slit 137 changes dependingon the wavelength of the light dispersed through the diffraction grating135 driven by the motor 134 and thus the time response becomes anoptical spectrum. Here, the time (light detection timing) can beconverted into the wavelength according to the angle of the diffractiongrating 135.

FIG. 9 is a time chart at the measurement time with the optical spectrumanalyzer shown in FIG. 8. FIG. 9( a) shows the pulse-like waveform ofmeasured light and FIG. 9( b) shows the waveform of a ramp wave signalfrom the motor controller 110 for performing wavelength sweep ofmeasured light.

FIG. 9( c) shows an electric signal output from the photodetector 138 inthe wavelength sweep state, FIG. 9( d) shows the waveform of a samplingclock supplied by the sampling clock generation section 152 to the A-Dconverter 150, and FIG. 9( e) shows the sampling waveform when output ofthe photodetector 138 is sampled in the A-D converter 150 in response tothe sampling clock.

FIG. 10 is a waveform drawing to schematically show the relationshipbetween the wavelength and the time. FIG. 10( a) is a three-dimensionalwaveform drawing to schematically show the relationship between thewavelength and the time, FIG. 10( b) is a two-dimensional waveformdrawing to show the characteristic of the wavelength with no timeinformation, and FIG. 10( c) is a schematic representation toschematically show the state of the wavelength in the time axisdirection.

Here, to mechanically rotate the diffraction grating 135 by the motor134, a number of light pulses of measured light are input as shown inFIG. 9( a) during one sweep (one crest of ramp wave in FIG. 9( b)).

Therefore, an optical spectrum averaged in terms of time in a state inwhich time information (see FIG. 10( c)) is lost is observed like thetwo-dimensional graph of the light strength relative to the wavelength(FIG. 10( b)). The sampling period is determined by the clock of the A-Dconverter 150.

Relevant arts to the optical spectrum analyzer for thus executingmeasurement are described in the following patent document 1, etc.:

[Patent document 1] Japanese Patent No. 3106979

[Patent document 2] Japanese Patent No. 3254932

Such an optical spectrum analyzer is used as a wavelength monitor of anoptical network, for example. In a next-generation optical network, datais relayed with a light signal intact without converting the lightsignal into an electric signal. Such an optical network uses atechnology called burst switching for switching a path at high speedaccording to the wavelength for transferring data. The time required forpath switching of the burst switching is about 1 ms and an opticalspectrum analyzer that can cope with high-speed wavelength switchingbecomes necessary.

However, in the mechanical wavelength sweep technique of rotating thediffraction grating 135 using the motor as shown in FIG. 8, a time ofabout one second is required with sweep span 1000 nm.

The three-dimensional graph shown in FIG. 10( a) is an example whereinthe wavelength of the measured light is switched instantaneously; in themechanical sweeping technique in the related art using a motor, the usercan observe only an optical spectrum averaged in a state in whichinformation in the time axis direction is lost like the two-dimensionalgraph in FIG. 10( b).

When the optical spectrum changes instantaneously as in FIG. 10( a), theoptical spectrum changes while the diffraction grating rotates, if thetiming is missed, there is also a possibility that erroneous informationsuch that an optical spectrum is not seen at all may be provided. If ahigh-speed scanner is used, instantaneous optical spectral change ofnano-order or less in a pulse cannot be captured.

SUMMARY

Embodiments of the present invention provide an optical spectrumanalyzer for enabling the user to observe an instantaneous opticalspectrum even if a mechanical sweep technique is used when the spectrumof measured light occurring repeatedly is measured.

One or more embodiments of the invention is as follows:

(1) The first aspect of the invention provides an optical spectrumanalyzer including an optical section for executing light dispersioninto a spectrum and wavelength sweep for input measured light,converting the measured light into an electric signal, and outputtingthe electric signal; a control section for controlling the wavelengthsweep of the optical section and outputting a sampling clock of a periodshifting from a cycle period of the measured light for each wavelengthof the wavelength sweep; and a measurement section for executingsequential sampling of the electric signal from the optical section foreach sampling clock.

(2) The second aspect of the invention provides an optical spectrumanalyzer including an optical section for executing light dispersioninto a spectrum and wavelength sweep for input measured light,converting the measured light into an electric signal, and outputtingthe electric signal;

a measurement section for processing the electric signal from theoptical section for each sampling clock; and

a control section for controlling the wavelength sweep of the opticalsection and changing the timing of sampling of the measurement sectionfor each period of the wavelength sweep.

(3) The third aspect of the invention, in the optical spectrum analyzeraccording to the first or second aspect of the invention, themeasurement section has a sampling head to which the electric signalfrom the optical section is input, the sampling head for samplingaccording to the sampling clock; and an A-D converter to which output ofthe sampling head is input, the A-D converter for converting analog datainto digital data according to the sampling clock.

(4) In the fourth aspect of the invention, the optical spectrum analyzeraccording to any of the first to third aspects of the invention furtherincludes a waveform display for producing three-dimensional waveformdisplay based on the measurement result of the measurement section.

(5) In the fifth aspect of the invention, in the optical spectrumanalyzer as claimed in any of the first to fourth aspects of theinvention, the optical section has a diffraction grating for executinglight dispersion of the measured light into a spectrum; and a motor forrotating the diffraction grating according to a command of the controlsection.

(6) In the sixth aspect of the invention, in the optical spectrumanalyzer according to any of first to fourth aspects of the invention,the optical section has a deflection section to which the measured lightis input, the deflection section for deflecting the measured lightaccording to a command of the control section; and a diffraction gratingon which diffraction light provided by the deflection section is madeincident, the diffraction grating for executing light dispersion into aspectrum.

(7) In seventh aspect of the invention, in the optical spectrum analyzeraccording to any of the first to sixth aspects of the invention, themeasured light is synchronized with the sampling clock.

(8) In the eighth aspect of the invention, in the optical spectrumanalyzer according to any of the first to sixth aspect of the invention,the measured light is repeated every frame period with the samplingclock and frame period synchronized with each other.

According to one or more embodiments of the invention described above,the following advantages can be provided:

In one or more embodiments of the invention described above, when theoptical section executes light dispersion into a spectrum and wavelengthsweep for input measured light, the measurement section executessequential sampling according to the sampling clock from the controlsection, of a period shifting from the cycle period of the measuredlight for each wavelength of the wavelength sweep.

Thus, sequential sampling is executed according to the sampling clock ofthe period shifting from the cycle period of the measured light for eachwavelength of the wavelength sweep, whereby when the spectrum ofmeasured light occurring repeatedly is measured, even if a mechanicalsweep technique is used, it is made possible to enable the user toobserve an instantaneous optical spectrum.

When the optical section executes light dispersion into a spectrum andwavelength sweep for input measured light, the measurement sectionexecutes sequential sampling according to the sampling clock whosetiming is changed from the control section for each period of thewavelength sweep. Thus, sequential sampling is executed according to thesampling clock whose timing is changed for each period of the wavelengthsweep, whereby when the spectrum of measured light occurring repeatedlyis measured, even if a mechanical sweep technique is used, it is madepossible to enable the user to observe an instantaneous opticalspectrum.

In the configuration described above, the sampling head executessampling according to the sampling clock, and the measurement sectionexecutes sequential sampling of output of the sampling head. Thesampling head for generating and sampling an electric signal of shortduration is installed and an electric signal generated in the opticalsection is sampled at the sampling head at high time resolution and thenA-D conversion of the output is executed and waveform display isproduced, whereby wide-band optical spectrum observation is madepossible independently of the performance of the A-D converter.

Further, in the configuration described above, three-dimensionalwaveform display is produced on the waveform display based on themeasurement result of the measurement section, whereby when the spectrumof measured light occurring repeatedly is measured, even if a mechanicalsweep technique is used, it is made possible to enable the user toobserve an instantaneous optical spectrum in a state in which change inthe wavelength is also contained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a functional block diagram to show the configuration of anoptical spectrum analyzer of a first embodiment of the invention;

FIGS. 2( a) to (d) are time charts to show the operation state of theoptical spectrum analyzer of the first embodiment of the invention;

FIG. 3 is a functional block diagram to show the configuration of anoptical spectrum analyzer of a second embodiment of the invention;

FIGS. 4( a) and (b) are schematic representation to schematically showprocessing of optical pulses in the first and second embodiments of theinvention;

FIGS. 5( a) to (d) are time charts to show the operation state of theoptical spectrum analyzer of the second embodiment of the invention;

FIG. 6 is a functional block diagram to show the configuration of anoptical spectrum analyzer of a third embodiment of the invention;

FIG. 7 is a functional block diagram to show the configuration of anoptical spectrum analyzer of a fourth embodiment of the invention;

FIG. 8 is a functional block diagram to show the configuration of anoptical spectrum analyzer in a related art;

FIGS. 9( a) to (e) are time charts to show the operation state of theoptical spectrum analyzer in the related art; and

FIGS. 10( a) to (c) are schematic representation to schematically showprocessing of optical pulses.

DETAILED DESCRIPTION

The best mode for carrying out the invention (embodiments) will bediscussed in detail with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a drawing to show the configuration of a measurement system ofan optical spectrum analyzer according to a representative Czerny-Turnermonochromator system as a first embodiment of the invention.

In FIG. 1, a measured light generation section 1 generates measuredlight and outputs the measured light to an incidence end of an opticalfiber 10.

The optical fiber 10 has the incidence end on which the measured lightfrom the measured light generation section 1 is incident, and transmitsthe incident measured light. An emission end of the optical fiber 10 onthe side of an optical section 130 described later is connected to anincidence slit 131 in the optical section 130 described later.

A control section 101 controls the sections of the optical spectrumanalyzer and particularly controls wavelength sweep in the opticalsection 130 in response to a synchronous signal of the measured lightgeneration section 1 and outputs a sampling clock for sequentialsampling of a period shifting from a cycle period of measured light foreach wavelength of wavelength sweep to a measurement section 140.

A CPU 102 controls the sections in accordance with a control program inthe control section 101 and gives a command to a signal generationsection 105, a motor controller 110, and a signal processing section 160a.

The signal generation section 105 receives a command of the CPU 102 inthe control section 101 and outputs a sampling clock to an A-D converter150 in synchronization with a synchronous signal from the measured lightgeneration section 1. The motor controller 110 outputs a ramp wavesignal of a predetermined waveform for driving a motor 134 for rotatinga diffraction grating 135, and has output connected to a divider 120.

The ramp wave signal of the predetermined waveform supplied from themotor controller 110 is input to the divider 120, which then divides theramp wave signal for output. The divided output is supplied to the motor134 and the signal processing section 160 a described later.

The optical section 130 executes light dispersion into a spectrum andwaveform sweeping for the measured light emitted from the output end ofthe optical fiber 10, converts the measured light into an electricsignal, and outputs the electric signal. It has the incidence slit 131,a concave mirror 132, the motor 134, the diffraction grating 135, aconcave mirror 136, an exit slit 137, and a photodetector 138.

The incidence slit 131 is placed in the proximity of the emission end ofthe optical fiber 10 for limiting a light flux so that the measuredlight having a predetermined emission angle, of the measured lightemitted from the emission end of the optical fiber 10 arrives at theconcave mirror 132.

The concave mirror 132 is a kind of collimator means for converting themeasured light having a spread angle from the optical fiber 10 incidentthrough the incidence slit 131 into collimated light, and reflects themeasured light converted into the collimated light toward thediffraction grating 135. The concave mirror 132 can also be replacedwith a collimator lens implemented as a convex lens, etc.

The motor 134 is a rotation drive source to which the ramp wave signalprovided by the divider 120 is supplied as a drive signal, and drivesthe diffraction grating 135 by rotation responsive to the ramp wavesignal.

The diffraction grating 135 is an optical element for executing lightdispersion of the diffraction angle responsive to the wavelength byreflection interference of a plurality of minute parallel grooves. Itreceives collimated measured light from the concave mirror 132 anddiffracts the dispersed measured light in the direction of the concavemirror 136.

The concave mirror 136 is a kind of light condensing means forcondensing dispersed diffraction light which is collimated light foreach wavelength from the diffraction grating 135. It condenses thediffraction light toward the photodetector 138. The concave mirror 136can also be replaced with a condensing lens implemented as a convexlens, etc.

The exit slit 137 is placed in the proximity of the light detection faceof the photodetector 138 for limiting a light flux so that the measuredlight of a predetermined wavelength, of the measured light dispersedthrough the diffraction grating 135 and condensed on the concave mirror136 toward the photodetector 138.

The photodetector 138 is a photoelectric conversion element fordetecting the measured light of the predetermined wavelength emittedthrough the exit slit 137 and outputting an electric signal, and hasoutput connected to the A-D converter 150 in the measurement section140.

The measurement section 140 conducts measurement by sequential samplingof an electric signal from the optical section 130 for each samplingclock, and is made up of the A-D converter 150 for executing A-Dconversion based on the sampling clock and the signal processing section160 a for performing signal processing of output of the A-D converter150.

The A-D converter 150 has input to which the output of the photodetector138 is connected, executes A-D conversion in response to a samplingclock from the signal generation section 105, outputs a digital signal,and has output connected to the signal processing section 160 a. Thesignal processing section 160 a performs signal processing of thedigital signal from the A-D converter 150, the sampling result of themeasured light of the predetermined wavelength, with the ramp wavesignal provided by the divider 120 as a trigger, generates waveforminformation of the spectrum component of the measured light, and outputsthe waveform information to a waveform display 170.

The waveform information output from the signal processing section 160 ais input to the waveform display 170 for producing various displays ofthe waveform information in the forms of graphs and numeric values as athree-dimensional waveform drawing, etc., about the optical spectrum ofthe measurement result of the measured light on a display screen.

The diffraction grating 135 can be varied at any desired angle by themotor 134 with the parallel axis to the grooves as the center. The motorcontroller 110 receives a command of the control section 101 andcontrols the motor 134 to vary the angle of the diffraction grating 135.The diffraction grating 135 diffracts diffraction light of only aspecific wavelength component determined by any desired angle from thecollimated light in the direction of the concave mirror 136. The concavemirror 136 forms the diffraction light on the exit slit 137. Only thewavelength component within the range of the breadth of the exit slit137 passes through the exit slit 137.

In FIG. 1, the measured light from the measured light generation section1 is incident on the optical section 130 from the emission end of theoptical fiber 10. At this time, the incidence slit 131 limits a lightflux so that the measured light having a predetermined emission angle,of the measured light emitted from the emission end of the optical fiber10 arrives at the concave mirror 132.

The measured light emitted from the emission end of the optical fiber 10and arriving at the concave mirror 132 arrives at the diffractiongrating 135 as collimated light. At this time, the measured lightdiffracted through the diffraction grating 135 is dispersed into aspectrum and is in a state in which the diffraction angle changes foreach wavelength.

Therefore, the wavelength of the measured light diffracted through thediffraction grating 135 and then condensed on the concave mirror 136 andemitted through the exit slit 137 changes in synchronization withrotation of the diffraction grating 135 and wavelength sweeping isrealized.

The measured light emitted through the exit slit 137 is detected at thephotodetector 138, which then executes photoelectric conversion of themeasured light into an electric signal. The electric signal detected andgenerated by the photodetector 138 is converted into a digital signal bythe A-D converter 150 driven according to a sampling clock from thesampling clock generation section 152.

The signal processing section 160 a performs signal processing of thedigital signal with a ramp wave signal as a trigger and generateswaveform information of the spectrum component of the measured light,and the waveform information is displayed on the waveform display 170.That is, the emission timing through the exit slit 137 changes dependingon the wavelength of the light dispersed through the diffraction grating135 driven by the motor 134 and thus the time response becomes anoptical spectrum. Here, the time (light detection timing) can beconverted into the wavelength according to the angle of the diffractiongrating 135.

Here, since a trigger is required for determining the measurement startpoint in the signal processing section 160 a, a control signal of themotor 134 synchronized with rotation of the diffraction grating 135 isdivided by the divider 120 and one is used as a trigger in the signalprocessing section 160 a.

FIG. 2 is a time chart of a signal waveform at the measurement time withthe optical spectrum analyzer in the first embodiment shown in FIG. 1.Here, spectrum measurement of measured light wherein wavelength changeof pulse in a frame period is repeated every frame period will bediscussed as an example.

FIG. 2( a) shows the pulse-like waveform of measured light and FIG. 2(b) shows the waveform of a ramp wave signal from the motor controller110 for performing wavelength sweep of measured light. FIG. 2( c) showsthe waveform of a sampling clock supplied by the signal generationsection 105 to the A-D converter 150, and FIG. 2( d) shows the samplingwaveform when output of the photodetector 138 is sequentially sampled inthe A-D converter 150 in response to the sampling clock.

FIG. 2( a) shows the waveform of input pulses of measured light, whereinthe period of one pulse is TO and three different pulses are generatedrepeatedly in a frame period Tc (frame pulse rate fc)

FIG. 10 shows a state of instantaneously switching from wavelength λ1 towavelength λ2 and again returning to λ1; the phenomenon sequence isassumed to be one frame (period: Tc=1/fc, fc: Frame pulse rate). Forsimplicity, here, three bits (three pulses) make up one frame; in fact,however, n bits (n: Integer) make up one frame. (1), (2), and (3) inFIG. 10( c) also correspond to (1), (2), and (3) in FIG. 2( a).

First, the control section 101 sets a ramp wave signal corresponding tothe wavelength λ1, drives the diffraction grating 135 through the motor134 from the motor controller 110, and controls the rotation angle ofthe diffraction grating 135 so as to allow the measured light of thewavelength λ1 to be emitted through the exit slit 137.

Here, to execute sequential sampling of the first (1) pulse (wavelengthλ1) in the frame period, more than one sampling is executed acrossframes with the ramp wave signal (FIG. 2( b)) kept in a given value forλ1. At this time, to execute sequential sampling, more than one samplingis executed (FIG. 2( c)) while a sampling clock is synchronized with thefirst (1) pulse (FIG. 2( c)→FIG. 2( a) (1)) and the sampling clockperiod (sampling period) Ts is shifted a little (1/Δf) from the frameperiod Tc.

In so doing, for the first (1) pulse of the wavelength λ1, the samplingresult across the frames is restored in the signal processing section160 a, whereby a similar waveform of the original pulses is obtained(FIG. 2( d)).

The control section 101 controls so as to execute sequential sampling ofthe second (2) pulse and sequential sampling of the second (3) pulsewith the ramp wave signal (FIG. 2( b)) kept in the given value for λ1.In the specific example, since the wavelength of the (2) pulse differsfrom λ1, the measured light subjected to wavelength sweep cannot beemitted through the exit slit 137 and is not detected at thephotodetector 138 and thus, in fact, a similar form of the originalpulses is not obtained. Since the wavelength of the (3) pulse is λ1, themeasured light is emitted through the exit slit 137 and is detected atthe photodetector 138 and a similar form of the original pulses isobtained by sequential sampling in the signal processing section 160 a.That is, such sampling is executed in sequence for each of other pulsesfor each time within one frame period.

Next, the control section 101 sets a ramp wave signal corresponding tothe wavelength λ2, drives the diffraction grating 135 through the motor134 from the motor controller 110, and controls the rotation angle ofthe diffraction grating 135 so as to allow the measured light of thewavelength λ2 to be emitted through the exit slit 137.

Also here, to execute sequential sampling of each of the first (1) pulsein the frame period, the second (2) pulse in the frame period, and thethird (3) pulse in the frame period, more than one sampling is executedacross frames with the ramp wave signal (FIG. 2( b)) kept in a givenvalue for λ2. At this stage, since the wavelength of each of the (1)pulse and the (2) pulse differs from λ2, the measured light cannot beemitted through the exit slit 137 and is not detected at thephotodetector 138 and thus a similar form of the original pulses is notobtained in the signal processing section 160 a. Since the wavelength ofthe (2) pulse is λ2, the measured light is emitted through the exit slit137 and is detected at the photodetector 138 and a similar form of theoriginal pulses is obtained by sequential sampling in the signalprocessing section 160 a.

Thus, in the signal processing section 160 a, a similar form of theoriginal pulse is obtained in sequence for the pulses different in timeand the pulses different in wavelength within the frame period. Thus, aswaveform information of the spectrum component of the measurement resultof the signal processing section 160 a, it is made possible to displaythe final detection result on the waveform display 170 like athree-dimensional graph having time axis information as in FIG. 10( a)rather than FIG. 10( b) in the related art.

In the specific example given above, sampling period Ts=1/(fc+Δf), butthe period may be set to N×Ts (N: Integer). Therefore, the diffractiongrating is rotated stepwise slowly in synchronization with the frameperiod, whereby three-dimensional waveform display is obtained asthree-dimensional information of the light strength relative to thewavelength and the time as shown in FIG. 10.

That is, in each wavelength, sequential sampling is executed for eachpulse and a similar form of the original pulse is obtained as describedabove. In this case, to rotate the diffraction grating 135 through themotor 134, the diffraction grating 135 need not be driven at high speedbecause it may be driven every (a plurality of frame periods requiredfor sequential sampling of one pulse)×(number of pulses within frameperiod) for sequential sampling, and the time axis information of thedetection result is not lost either.

Thus, the sequential sampling method is used, whereby in the signalprocessing section 160 a, instantaneous spectral change in a pulse asshown in FIG. 10( a) rather than simply averaged optical spectrum (FIG.10( b)) can be captured. This does not depend on the sweep speed andthus can also be sufficiently applied to machine sweep of rotating thediffraction grating 135 with the motor 134 at low sweep speed. It canalso be applied to the case where any other sweep means is used.

Although the configuration wherein the measured light generation section1 outputs a synchronous signal to the signal generation section 105 isshown, a synchronous signal may be output from the signal generationsection 105 to the measured light generation section 1. In short, themeasured light generation section 1 and the signal generation section105 may be synchronized with each other.

Measurement of measured light different in wavelength of pulses within aframe is shown by way of example, but the invention is not limited toit; burst or packets maybe used in place of pulses. This means thatmeasurement of measured light different in wavelength of burst orpackets within a frame may be executed. In short, limitation to measuredlight is not made.

Not only in measured light with change in the wavelength of a pulse or apulse string, but also in repetition of a pulse having a givenwavelength, minute wavelength fluctuation of a pulse or a pulse stringand change in a light strength distribution can be observed.

The example wherein the sampling clock is synchronized every frameperiod is used, in which case the pulse time and wavelength changewithin a frame can be captured. If the sampling clock is notsynchronized with the frame period, although each pulse in the framecannot be observed, statistical data of the pulses can be obtained andthus the mode is effective for waveform evaluation.

Second Embodiment

FIG. 3 is a block diagram to show the configuration of a measurementsystem of an optical spectrum analyzer according to a representativeCzerny-Turner monochromator system as a second embodiment of theinvention.

Components identical with those in FIG. 1 are denoted by the samereference numerals in FIG. 3 and will not be discussed again. The secondembodiment in FIG. 3 differs from the first embodiment in FIG. 1 in thatit has a time delay device 107 to which a control signal from a controlsection 101 and a sampling clock from a signal generation section 105are input, the time delay device 107 for supplying output to an A-Dconverter 150.

In the first embodiment described above, a different wavelength is sweptgradually in such a repetitive manner that a plurality of pulses aresequentially sampled within the same wavelength and then the wavelengthis switched to a different wavelength and sequential sampling isexecuted in the wavelength. This is represented as an image as in 1, 2,3, . . . in FIG. 4( a).

In the second embodiment, unlike the first embodiment, while wavelengthsweep is executed about the same point of pulse, the point is shiftedgradually as shown in 1, 2, 3, . . . in FIG. 4( b).

FIG. 5 is a time chart at the operation time of the second embodiment.In the optical spectrum analyzer of the configuration shown in FIG. 3,measured light and a sampling clock need to be synchronized with eachother as in the configuration in FIG. 1; in the second embodiment,however, the sampling clock frequency is fc rather than fc+Δf.

Accordingly, from the viewpoint of pulses, wave sweep is always executedat the same timing of pulse, as shown in FIG. 5. Therefore, to seechange in an optical spectrum based on the time in the pulse or see theoptical spectrum of another pulse, the time delay device 107 becomesnecessary for shifting the timing. It is necessary to send a controlsignal from the control section 101 to the time delay device 107 inagreement with the wavelength sweep period for shifting the timing.Therefore, upon reception of a signal from a motor controller forcontrolling wavelength sweep, the control section 101 sends a controlsignal to the time delay device 107 for gradually shifting the timing toanother pulse, as shown in FIG. 3.

FIGS. 5( a) to (d) shows the state in which wavelength sweep is executedjust at the peaks of pulses (1) and (2). The sampling timing is shiftedlittle by little by the time delay device 107, whereby three-dimensionalwaveform display as three-dimensional information of the light strengthrelative to the wavelength and the time can be produced as a measurementsection 140 performs signal processing as with the first embodiment.

Since sequential sampling is also used in the second embodiment, adiffraction grating 135 is slowly rotated, whereby in a signalprocessing section 160 a, instantaneous spectral change in a pulse asshown in FIG. 10( a) rather than simply averaged optical spectrum (FIG.10( b)) can be captured. This does not depend on the sweep speed andthus can also be sufficiently applied to machine sweep of rotating thediffraction grating 135 with a motor 134 at low sweep speed. It can alsobe applied to the case where any other sweep means is used.

Third Embodiment

FIG. 6 is a block diagram to show the configuration of an opticalspectrum analyzer with a sequential sampling method applied to anoptical system using an acousto-optic deflector (AOD) in place of adiffraction grating rotated by a motor as means for executing wavelengthsweep as a third embodiment of the invention.

An optical section 130 will be discussed centering on the differencesfrom the first and second embodiments described above.

In the third embodiment, an optical section 130 uses an acousto-opticdeflector (AOD) 130 b through which the propagation angle of diffractionlight changes by an electric signal as an example of deflection sectionin place of the diffraction grating 135 of the motor 134.

A voltage-controlled oscillator (VOC) 114 for receiving a ramp wavesignal, generating a radio frequency (RF) signal of the frequencyresponsive to the ramp wave signal, and supplying the RF signal to theAOD 103 b is provided for deflecting the propagation angle of thediffraction light of the AOD 103 b.

Although the motor controller generates a ramp wave signal in the firstembodiment, etc., a waveform generation section 112 generates a rampwave signal under the control of a control section 101 as an equivalentfunction.

In the optical section 130 shown in FIG. 6, a collimator lens 130 a isused in place of the concave mirror 132 and a condensing lens 130 c isused in place of the concave mirror 136 because of the relation ofplacement, but they serve optical functions equivalent to those of theconcave mirrors in the first and second embodiments.

Measured light is output to space through an optical fiber 10 and isconverted into collimated light through the collimator lens 130 a. Whenthe collimated light of the measured light is made incident on the AOD130 b, the propagation angle of the diffraction light changes accordingto the RF frequency of VCO output from the VCO 114 for generating thefrequency responsive to the voltage of the ramp wave signal.

Therefore, as the ramp wave signal is input to the VCO 114, thewavelength emitted through an exit slit 137 is swept with a diffractiongrating fixed mechanically.

Thus, although the internal configuration of the optical section 130differs, the manner in which some control signal is input for executingwavelength sweep and photodetector output is sampled is the same as thatin the first and second embodiments, so that the sequential samplingmethod can also be applied to the wave sweep method using the AOD 130 b.

That is, the sequential sampling method shown in the time chart of FIG.2 in the first embodiment and the sequential sampling method shown inthe time chart of FIG. 5 in the second embodiment can be applied intactby supplying a ramp wave signal to the VCO 114.

Since sequential sampling is also used in the third embodiment, the AOD130 b is slowly driven, whereby three-dimensional information of thelight strength relative to the wavelength and the time as shown in FIG.10( a) can be produced.

That is, the sequential sampling method is used, whereby as the waveforminformation of the spectrum component of the processing result of asignal processing section 160 a, it is made possible to display thefinal detection result on a waveform display 170 like athree-dimensional graph having time axis information as in FIG. 10( a)rather than simply averaged optical spectrum as in FIG. 10( b) in therelated art.

Since the sequential sampling method described above does not depend onthe sweep speed, the components need not be driven at high speed and thesequential sampling method can be sufficiently applied. It can also beapplied to the case where any other sweep means is used.

In the third embodiment, any of various beam deflection elements of agalvano scanner, a polygon mirror, an MEMS (Micro Electro MechanicalSystems) mirror, etc., may be adopted in place of the AOD 130 b as thedeflection section for executing wavelength sweep.

In the monochromator, a representative Czerny-Turner optical system isshown in FIG. 1, but any of various optical placements of Ebert type,Littrow type, Monk-Gillieson type, etc., may be used. In this case, themethod shown in the time chart of FIG. 2 in the first embodiment and themethod shown in the time chart of FIG. 5 in the second embodiment canalso be applied.

Fourth Embodiment

FIG. 7 is a block diagram to show the configuration of an opticalspectrum analyzer of a sequential sampling method using a sampling headfor more speeding up (higher time resolution) as a fourth embodiment ofthe invention.

The fourth embodiment will be discussed centering on the differencesfrom the first and second embodiments described above. In an opticalspectrum analyzer of the fourth embodiment, a sequential samplingmeasurement system wherein a sampling head 142 is placed at the stagepreceding an A-D converter 150 is applied.

Generally, a photodetector 138 has a wider frequency band than the A-Dconverter 150. Therefore, the response speed of the whole of the opticalspectrum analyzer is determined depending on the A-D converter.

Then, the sampling head 142 for generating and sampling an electricsignal of short duration as used with a sampling oscilloscope, etc., isplaced between output of the photodetector 138 and input of the A-Dconverter 150 and samples the output of the photodetector 138 at hightime resolution, the A-D converter 150 executes A-D conversion of theoutput, and waveform display is produced. According to theconfiguration, optical spectrum observation with a band of 100 GHz ormore is made possible and even observation of a 1-bit instantaneousoptical spectrum of a high-speed communication optical signal is madepossible.

For wavelength sweep in an optical section 130, the method shown in thetime chart of FIG. 2 in the first embodiment and the method shown in thetime chart of FIG. 5 in the second embodiment can be applied intact.Although not shown, the AOD 130 b of the third embodiment can also beapplied to wavelength sweep.

Since sequential sampling is also used in the fourth embodiment,three-dimensional information of the light strength relative to thewavelength and the time as shown in FIG. 10( a) can be produced. Thatis, the sequential sampling method is used, whereby as the waveforminformation of the spectrum component of the processing result of asignal processing section 160 a, it is made possible to display thefinal detection result on a waveform display 170 like athree-dimensional graph having time axis information as in FIG. 10( a)rather than simply averaged optical spectrum as in FIG. 10( b) in therelated art. Since the sequential sampling method described above doesnot depend on the sweep speed, the components need not be driven at highspeed and the sequential sampling method can be sufficiently applied. Itcan also be applied to the case where any other sweep means is used. Thesampling head 142 is placed at the stage preceding the A-D converter150, whereby it is also made possible to handle a high-speedcommunication optical signal exceeding the response speed of the A-Dconverter 150.

1. An optical spectrum analyzer comprising: an optical section forexecuting light dispersion into a spectrum and wavelength sweep forinput measured light, converting the measured light into an electricsignal, and outputting the electric signal; a control section forcontrolling the wavelength sweep of said optical section and outputtinga sampling clock of a period shifting from a cycle period of themeasured light for each wavelength of the wavelength sweep; and ameasurement section for executing sequential sampling of the electricsignal from said optical section for each sampling clock.
 2. An opticalspectrum analyzer comprising: an optical section for executing lightdispersion into a spectrum and wavelength sweep for input measuredlight, converting the measured light into an electric signal, andoutputting the electric signal; a measurement section for processing theelectric signal from said optical section for each sampling clock; and acontrol section for controlling the wavelength sweep of said opticalsection and changing the timing of sampling of said measurement sectionfor each period of the wavelength sweep.
 3. The optical spectrumanalyzer as claimed in claim 1 wherein said measurement section has: asampling head to which the electric signal from said optical section isinput, the sampling head for sampling according to the sampling clock;and an A-D converter to which output of the sampling head is input, theA-D converter for converting analog data into digital data according tothe sampling clock.
 4. The optical spectrum analyzer as claimed in claim2 wherein said measurement section has: a sampling head to which theelectric signal from said optical section is input, the sampling headfor sampling according to the sampling clock; and an A-D converter towhich output of the sampling head is input, the A-D converter forconverting analog data into digital data according to the samplingclock.
 5. The optical spectrum analyzer as claimed in claim 1 furthercomprising: a waveform display for producing three-dimensional waveformdisplay based on the measurement result of said measurement section. 6.The optical spectrum analyzer as claimed in claim 2 further comprising:a waveform display for producing three-dimensional waveform displaybased on the measurement result of said measurement section.
 7. Theoptical spectrum analyzer as claimed in claim 1 wherein said opticalsection has: a diffraction grating for executing light dispersion of themeasured light into a spectrum; and a motor for rotating the diffractiongrating according to a command of said control section.
 8. The opticalspectrum analyzer as claimed in claim 2 wherein said optical sectionhas: a diffraction grating for executing light dispersion of themeasured light into a spectrum; and a motor for rotating the diffractiongrating according to a command of said control section.
 9. The opticalspectrum analyzer as claimed in claim 1 wherein said optical sectionhas: a deflection section to which the measured light is input, thedeflection section for deflecting the measured light according to acommand of said control section; and a diffraction grating on whichdiffraction light provided by the deflection section is made incident,the diffraction grating for executing light dispersion into a spectrum.10. The optical spectrum analyzer as claimed in claim 2 wherein saidoptical section has: a deflection section to which the measured light isinput, the deflection section for deflecting the measured lightaccording to a command of said control section; and a diffractiongrating on which diffraction light provided by the deflection section ismade incident, the diffraction grating for executing light dispersioninto a spectrum.
 11. The optical spectrum analyzer as claimed in claim 1wherein the measured light is synchronized with the sampling clock. 12.The optical spectrum analyzer as claimed in claim 2 wherein the measuredlight is synchronized with the sampling clock.
 13. The optical spectrumanalyzer as claimed in claim 1 wherein the measured light is repeatedevery frame period with the sampling clock and frame period synchronizedwith each other.
 14. The optical spectrum analyzer as claimed in claim 2wherein the measured light is repeated every frame period with thesampling clock and frame period synchronized with each other.